This class template implements a generator for R-MAT graphs [CZF04],
suitable for initializing an adjacency_list or other graph structure
with iterator-based initialization. An R-MAT graph has a scale-free
distribution w.r.t. vertex degree and is implemented using
Recursive-MATrix partitioning.

Constructs an R-MAT generator iterator that creates a graph with n
vertices and m edges. Inside the scalable_rmat_iterator
processes communicate using pg to generate their local edges as
defined by distrib. a, b, c, and d represent the
probability that a generated edge is placed of each of the 4 quadrants
of the partitioned adjacency matrix. Probabilities are drawn from the
random number generator gen. Vertex indices are permuted to
eliminate locality when permute_vertices is true.